Here’s a little what Aquinas had to say about beauty:
Beauty and goodness in a thing are identical fundamentally; for they are based upon the same thing, namely, the form; and consequently goodness is praised as beauty.
But they differ logically, for goodness properly relates to the appetite (goodness being what all things desire); and therefore it has the aspect of an end (the appetite being a kind of movement towards a thing).
On the other hand, beauty relates to the cognitive faculty; for beautiful things are those which please when seen. Hence beauty consists in due proportion; for the senses delight in things duly proportioned, as in what is after their own kind—because even sense is a sort of reason, just as is every cognitive faculty. Now since knowledge is by assimilation, and similarity relates to form, beauty properly belongs to the nature of a formal cause.
ST I, Q. 5 A. 4